Secondary batteries such as lithium secondary batteries and nickel-hydrogen batteries are gaining importance as power sources installed in vehicles that employ electricity as a driving source, and in electric devices such as personal computers, cell phones and the like. In particular, lithium secondary batteries are expected to be used as preferred high-output power sources in vehicles, thanks to their light weight and the high energy density that they afford.
In a typical construction, one such lithium secondary battery comprises an electrode collector, and provided on the surface thereof, electrode active material layers (specifically, a positive electrode active material layer and a negative electrode active material layer) that are capable of reversibly storing and releasing lithium ions. In the case of a positive electrode, for instance, a positive electrode active material layer is formed by coating a positive electrode collector with a paste-like composition for forming positive electrode active material layers (the term paste-like composition includes slurry-like compositions or ink-like compositions; likewise hereafter), wherein the paste-like composition is prepared by mixing, for instance, a positive electrode active material, such as a lithium-transition metal complex oxide, a high-conductivity material powder (conductive material), and a binder, in an appropriate solvent.
The solvent used in the mixture for preparing the above paste-like composition may be an aqueous solvent. As compared with a paste-like composition obtained using an organic solvent (hereafter, “non-aqueous paste” or “non-aqueous composition”), a paste-like composition obtained using an aqueous solvent (hereafter, “aqueous paste” or “aqueous composition”) is advantageous overall in terms of environmental burden, since the aqueous paste utilizes little organic solvent and generates thus little industrial waste associated therewith. Moreover, no equipment is required for treating that waste, and thus no treatment costs are incurred.
Depending on the positive electrode active material, however, the pH of the aqueous paste tends to increase on account of the high reactivity with water of the positive electrode active material (for instance, an oxide of composition formula LiNiO2 in the case of a lithium-nickel complex oxide). Compounds of high electric resistivity (for instance, oxides or hydroxides) are formed on the surface of a metallic positive electrode collector (for instance, of aluminum) when an aqueous paste of high pH is coated onto the surface of such a collector. The positive electrode collector may corrode as a result, which may cause the internal resistance of the battery to increase.
Patent Documents 1 and 2 disclose conventional technologies relating to battery electrodes using such aqueous pastes. In Patent Document 1, formation of high electric resistivity compounds, which give rise to corrosion of positive electrode collectors, can be prevented by interposing a conductive layer, containing a conductive material and formed out of a non-aqueous paste, between a positive electrode collector and an active material formed out of an aqueous paste. Patent Document 2 discloses the feature of equalizing current collection by a collector through formation of a carbon film, having a specific lattice plane spacing, on a collector made of aluminum.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-4739    Patent Document 2: Japanese Patent Application Laid-open No. 10-106585
However, lithium secondary batteries having a structure comprising a stack of two layers prepared out of paste-like compositions of mutually different character, i.e. aqueous and non-aqueous, as in the case of the above-described technologies, are susceptible to loss of adherence between layers and drops in conductivity after long-term use.
A lithium secondary battery may conceivably be used over long periods of time under repeated high-rate charge and discharge (high-speed charge and discharge). Typical examples of lithium secondary batteries that may be used in that state include, for instance, lithium secondary batteries employed as power sources in vehicles (typically, automobiles, in particular hybrid automobiles and electric automobiles). Lithium secondary batteries under repeated high-rate charge and discharge are subjected to substantial load. This raises the concern of delamination between the two stacked layers prepared out of paste-like compositions having dissimilar characters i.e. aqueous and non-aqueous.